This project proposes to form three-dimensional crystals of the chloride exchange channel (Band 3), which is the most abundant protein of the erythrocyte membrane. This protein appears to be structurally related to the chloride channel which has been implicated in a causal relationship with cystic fibrosis. The protein will be isolated and purified from human erythrocytes, and the glycosylated and unglycosylated forms separated. Microdialysis and drop fusion methods of crystallization of the detergent-solubilized form will be used. In addition, batch co- crystallizations with lipid will be done for the 17K hydrophobic proteolytic fragment which represents the channel portion of this membrane protein. Crystals formed will be subjected to analysis by X-ray crystallography in order to provide insight into the molecular basis of the protein's functional properties.